NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

This specification defines two new elements, <rtcp-fb/> and
<rtcp-fb-trr-int/>, that can be inserted in the
<description/> or the <payload-type/> elements of
Jingle RTP Sessions (XEP-0167) [3]. The presence of any of these elements in a
content's description means that the RTP/AVPF profile should be used
for the whole content. If any of these elements are inside the
<payload-type/> element, the parameters specified apply only to
that payload type, if they are directly inside the
<description/> tag, then the specified parameters apply to the
whole content.

Any type or subtype of feedback message that requires extra
parameters in the a=b form can use the <parameter/> element to
describe it. Any other form of parameter can be stored as the 'key' attribute in a parameter element with an empty value.

Note: this overlaps with the subtype attribute. If there is only one parameter, use the subtype. The only known example where this is required is ccm.

The element <rtcp-fb-trr-int/> is used to specify the minimum
interval between two Regular (full compound) RTCP packets in
milliseconds for this media session. It corresponds to the
"a=rtcp-fb:* trr-int" line in SDP. The attributes of the
<rtcp-fb-trr-int/> element are:

Feedback messages are negotiated along side the codecs. They follow
the same Offer/Answer mechanism based on SDP Offer/Answer. The
initiator signals which feedback messages it wants to send or
receive in the the <session-initiate/> iq stanza. If the
responder does not understand the type or subtype of a feedback
message, it MUST remove the element from the reply. If the responder
does not wish to provide or receive some kind of feedback, it MUST
remove the relevant element. It MUST then send the remaining
elements it wants to keep as-is without modifying them in the
<session-accept/>

To conform with the negotiation rules outlined in RFC 4585 Section 4,
the responder MUST send any <rtcp-fb/> element as-is if it
accepts it. It MUST NOT change any parameter. It MUST NOT add any
<rtcp-fb/> element that was not offered by the initiator. It
MUST NOT modify the 'value' of any <rtcp-fb-trr-int/>
element. It can only remove the <rtcp-fb-trr-int/> element or
reject the content. If all the feedback messages are removed but the
responder wants to stay in the RTP/AVPF profile, it MUST put a
<rtcp-fb-trr-int/> element with the same 'value' that it
received from the intiator, if the initiator did not provide a
<rtcp-fb-trr-int/> element, then this value is "0".

Example negotiation where the initiator requests Packet Loss
Indications (pli) as defined in RFC 4585 on both H.263 and H.264,
but also requests Slice Loss Indications for H.264 with a minimum
interval between regular full compound RTCP packets of 100 milliseconds.

The <rtcp-fb/> element maps to the a:rtcp-fb= SDP line with
the exception of the 'trr-int' parameter which is mapped into it's
own element (<rtcp-fb-trr-int/>) in XMPP. The payload types
are also not explicitly written in the <rtcp-fb/> and
<rtcp-fb-trr-int/> elements. Instead, each payload type has its own
set of <rtcp-fb/> and <rtcp-fb-trr-int/> elements if
they do not apply to the whole content.

Example conversion of a sample fragment of a SDP containing an
audio session using the RTP/AVP profile for audio and the RTP/AVPF
profile for video:

To advertise its support for Extended RTCP Feedback in Jingle RTP
Sessions and a minimum interval between regular RTCP packets, when
replying to Service Discovery (XEP-0030) [4] information requests an entity MUST return the
following features:

URNs for any version of this protocol that the entity
supports -- e.g., "urn:xmpp:jingle:apps:rtp:rtcp-fb:0" for the
current version

If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.

Appendix C: Legal Notices

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

There exists a special venue for discussion related to the technology described in this document: the <jingle@xmpp.org> mailing list.

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

5. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.